Conventionally, the extraction sheets used as the material of filters or bags for extracting beverages such as black tea, green tea, and coffee, liquid food such as soup stock, drugs such as Kampo products, or quasi drugs such as bath salts are made from a nonwoven or a woven fabric made of resin, or paper in order to increase the rate of extraction and decrease the cost. Especially in recent years, nonwoven fabrics with small basis weight have tended to be preferred owing to their improved appearance such as transparency and luster.
However, since an extraction sheet comprising a nonwoven fabric with small basis weight is generally thin and flexible, the extraction sheet is difficult to handle upon being processed into extraction filters, and thus has a problem of low machinability. In the manufacture of the extraction filters by an elongated extraction sheet comprising a nonwoven fabric, for example, the extraction sheet is so flexible that, when fed and conveyed at a high speed on a manufacturing line, the extraction sheet may fail to follow a guide member placed on the manufacturing line and meander right and left with respect to a conveyance direction. This forms inclined cutting or sealing lines, causing defective products.
In addition, since an extraction bag in the form of a pillow filled with an extraction material such as tea leaves generally swells non-uniformly with the extraction material, the extraction bags, when stacked, tends to become unsteady. In particular, since an extraction bag comprising a conventional nonwoven fabric is flexible, is easily deformed, and has a smooth surface, the stacked extraction bags are unsteady, so that the stacked extraction bags easily slip down from the upper part of the stack. Hence, it is difficult to convey the extraction bags in stack or house them in an external container, which causes a problem of poor handling property.
In contrast, an extraction sheet made of paper generally has a characteristic of being creasable. Therefore, in the manufacture of the extraction filters, for example, the guide members are placed on the manufacturing line and the extraction sheet made of paper is folded in half or otherwise bent to form a crease line along the conveyance direction of the manufacturing line, enabling the extraction sheet to be in a tight and straight condition. Thus, such an extraction sheet can be prevented from meandering.
In addition, in the manufacture of an extraction bag in the form of a pillow formed with the extraction sheet made of paper, the crease line can be appropriately formed to provide the extraction bag having improved shape retention. Thus, the extraction bags, even when stacked, can hardly slip down.
Therefore, the present inventors envisaged that if even the extraction sheet comprising a nonwoven fabric made of resin can have such a characteristic of being creasable as exhibited by paper, then the extraction sheet can be prevented from meandering in the manufacture of the extraction filters and also provide the extraction bag having improved shape retention and thus enhanced handling property.
Japanese Patent Application Publication No. 2011-157118 (Patent Literature 1) as prior-art relevant to an extraction sheet comprising a nonwoven fabric of a polylactic acid-based resin describes a filter for food comprising a laminated nonwoven fabric integrated by thermocompression bonding a spunbonded nonwoven fabric and a meltblown nonwoven fabric each of the polylactic acid-based resin. It is also described that since this filter for food has small space among fibers, the filter can prevent powder leakage, has high rigidity, and a stabilized mechanical strength and sealing strength.
Although, the filter for food described in Patent Literature 1 includes the layer of the meltblown nonwoven fabric comprising the fibers of the polylactic acid-based resin, the filter for food is nevertheless hardly creasable due to the high crystallinity of the resin forming the meltblown nonwoven fabric layer. Hence, this filter for food has a problem of failing to enhance machinability by forming the crease line.